﻿///////////////////////////////////////////////////////////////////////////////
//
//  This source is part of the  "Managed Task Framework.NET Runtime"  (MTF.NET Runtime).
//
//
//  Copyright (c) 2011, Multicore Consulting Limited (UK)
//  All rights reserved.
//  http://www.multicoreconsulting.co.uk
//
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions
//  are met:
//
//  -  Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//  -  Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in
//     the documentation and/or other materials provided with the
//     distribution.
//  -  Neither the name of Multicore Consulting Limited nor the names of
//     its contributors may be used to endorse or promote products
//     derived from this software without specific prior written
//     permission.
//
//  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
//  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
//  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
//  FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
//  COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
//  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
//  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
//  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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//  ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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///////////////////////////////////////////////////////////////////////////////

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace MTFOrg.MTF.MTFRuntime
{
    /// <summary>
    /// Implements a spin lock synchronization primitive.
    /// </summary>
    public class SpinLock
    {
        /// <summary>
        /// Constant used to identify lock with no owner.
        /// </summary>
        const int NO_OWNER = -1;

        /// <summary>
        /// Semaphore used for waiting when the lock is not available.
        /// </summary>
        Semaphore m_Sem;

        /// <summary>
        /// Identifies the current owner of the lock.
        /// </summary>
        int m_Owner;

        /// <summary>
        /// Number of heavyweight waiters for the lock.
        /// </summary>
        int m_HeavyWaiters;

        /// <summary>
        /// Default constructor.
        /// </summary>
        public SpinLock()
        {
            m_Sem = new Semaphore(0, int.MaxValue);
            m_HeavyWaiters = 0;
            m_Owner = NO_OWNER;
        }

        /// <summary>
        /// Acquires a lock.
        /// </summary>
        public void Lock()
        {
            int threadId = Thread.CurrentThread.ManagedThreadId;

            // Increment heavy waiters as this thread could become a heavy waiter
            Interlocked.Increment(ref m_HeavyWaiters);

            while (true)
            {
                // Attempt m_SpinCount times to acquire the fast lock
                if (Interlocked.CompareExchange(
                    ref m_Owner,
                    threadId,
                    NO_OWNER) == NO_OWNER)
                {
                    break;
                }

                m_Sem.WaitOne();
            }

            // Decrement heavy waiters
            Interlocked.Decrement(ref m_HeavyWaiters);
        }

        /// <summary>
        /// Unlocks the spin-lock.
        /// </summary>
        public void Unlock()
        {
            int threadId = Thread.CurrentThread.ManagedThreadId;

			// Release the lock
			while (true)
			{
				if (Interlocked.CompareExchange(ref m_Owner, NO_OWNER, threadId) == threadId)
					break;
			};

            // Release a heavy waiter if there is one
            if (m_HeavyWaiters > 0)
            {
                m_Sem.Release(1);
            }
        }
    }
}
